Gerhard M. Sessler and James E. West are generally credited with the discovery in 1962 that certain plastic dielectric materials can be employed to advantage in electret microphones. Their electret permanently retained electrostatic charge and provided sufficiently low output impedance to operate with then conventional transistor amplifiers.
A number of manufacturers since 1962 have commercially made available compact packages wherein a semi-conductor amplifier and an electret microphone, in combination, provide an electrical signal in the audio frequency range.
In the movement toward further miniaturization of the electret microphone and amplifier combination, however, manufacturers have been impeded in their efforts to employ integrated circuit technology. The problem they have faced is that of applying integrated circuit technology to the generally high impedance (low capacitance) of small electret microphones. Also, it is especially useful of an integrated circuit amplifier that it provide an overall gain to the output of the electret and provide an output impedance several orders of magnitude lower than the impedance of the electret.
One integrated circuit amplifier known commercially as the SL793 capacitance microphone amplifier manufactured by Plessy Semi-conductors is capable of operation with certain electret microphones. In particular, the SL793 amplifier employs series voltage feedback techniques to provide an input impedance of six megohms, a nominal voltage gain of 20 dB, and a maximum output impedance of twenty five ohms. The problem with the Plessy amplifier is that it may be only employed with relatively high capacitance electret microphones if good low frequency response is to be maintained. For example, to achieve a low-frequency cut off of 100 hertz, a microphone with a capacitance of at least two hundred picofarads must be used.
In the design of telephone station apparatus, however, the capacitance presented by small electret microphones may be on the order of one to ten picofarads. In order to provide equivalent low frequency rolloff from the electret microphone, the input resistance of an appropriate amplifier must vary inversely with the microphone capacitance. Thus, series voltage feedback techniques suggested by Plessy have proven insufficient. It is urged that a maximum limitation of that technology is a fifty-megohm input impedance in the voice frequency band of interest.
In addition, electrical noise in the telephone station environment becomes a significant problem at the output of the amplifier. Thus, an integrated circuit amplifier for application with miniature microphones should have a sufficiently low output impedance and provide sufficient gain to permit signal transmission in an electrically noisy environment for considerable distances.